Most of the people on the forum simply go by how long their hair takes to dry without product in it. My hair will be dry in under two hours with no product, so I consider myself to have high porosity. Others may find that their hair takes 4-6 hours to dry, or even longer, which is low porosity. Anything in the middle would be normal porosity. (These are just guesstimates.)

If you want a more scientific way, I think the only way to know for sure is to get a hair anaylsis from GooseFootprints.